Put the reason for "revalidation policy" calculation to trace events

Also,
 - Put the name of the result policy, not the integer representing
   the policy, to the events.
 - Gather RESOURCE_LOADING_DVLOG to reduce code duplication.

Bug: 937173
Change-Id: Ibf19e2b853fbbe6b100f847c3ed3cc63e21e02e7
Reviewed-on: https://chromium-review.googlesource.com/c/1496265Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636785}

third_party/blink/renderer/platform/loader/fetch/resource_fetcher.h

@@ -308,14 +308,18 @@ class PLATFORM_EXPORT ResourceFetcher
       const FetchParameters&,
       const Resource& existing_resource,
       bool is_static_data) const;
+  static const char* GetNameFor(RevalidationPolicy policy);
   // Determines a RevalidationPolicy given a FetchParameters and an existing
   // resource retrieved from the memory cache (can be a newly constructed one
   // for a static data).
-  RevalidationPolicy DetermineRevalidationPolicyInternal(
-      ResourceType,
-      const FetchParameters&,
-      const Resource& existing_resource,
-      bool is_static_data) const;
+  // The first member of the returned value is the revalidation policy. The
+  // second member of the returned value is a string explaining the reason for
+  // trace events. Its extent is unbound.
+  std::pair<RevalidationPolicy, const char*>
+  DetermineRevalidationPolicyInternal(ResourceType,
+                                      const FetchParameters&,
+                                      const Resource& existing_resource,
+                                      bool is_static_data) const;
 
   void MakePreloadedResourceBlockOnloadIfNeeded(Resource*,
                                                 const FetchParameters&);